Amplifying system



Patented June 29, 1943 UNITED STATES PATENT OFF-ICE siurmrmo SYSTEM Alfred A. Roetken, Madison, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 26,1041, set-aunt. 390,450

Claims.

This invention relates to amplifying systems and the object of the invention is an amplifying system in which the amplitude of the signal waves transmitted is automatically varied in a cyclic manner.

- Amplitude modulated waves of this type are useful for various purposes and particularly for producing tremolo effects in connection with electronic organs or similar instruments.' 5

number of tremolo mechanisms have been suggested heretofore but these have not been entirely satisfactory for various reasons. Some of these, for example, provide no means for producing a tremolo efiect which varies with frequency and others involve the use of electrical contacts for this purpose and such contacts may choosing the transformers or other circuit elements to give the desired response or by insert- .ing a suitable network 18 in any convenient part introduceobjectionable noise after aperiod of use.

According to this invention an amplifier is provided with a negative feedback path and the amount of feedback is varied cyclically at the" amplitude and frequency required to produce the desired amplitude modulations in the outpiit circult. In the preferred embodiment an'amplifier is used in .the feedback path and the feedback is varied by means of an oscillator which varies the gain of the feedback amplifier. In cases where the amplitude of the modulations should vary with the frequency of the signals being transmitted, the feedback path has a transmission characteristic which varies with frequency in the manner required to give the tremolo effect desired. If, as in the case of electronic organs, the tremolo is to increase in amplitude with frequency, the gain reduction due to feedback may be a maximum at the upper limiting frequency and, by means of networks inthe" feedback path, be tapered oi'f to zero at some lower frequency below which no tremolo effect is desired. \By giving the main signal amplifier when operating without feedback 9. transmission characteristic which varies, with frequency in the same manner as the transmission through the network of 'the feedback path, the desired tremolo effect is obtained with uniform transmission over the whole band transmitted.

In the drawings Fig. 1 is an amplifying system according to the invention; and r Fig. 2 is analternate circuit using push-pull signal and feedback amplifiers. a,

The system shown in 1 comprises a main signal amplifier i,-a feedback. amplifier 2. an

oscillator 3 and a conventional power supply set 4. fbignals from the I, which in'the case the tube It in the feedback amplifier through a condenser 12 and a resistor II in the usual manner. The output of the tube It is connected to the input of the tube ll and-the output of the latter tube is applied through a condenser II and a resistor I as negative feedback to the grid of the tube 1 in the main amplifien This feedback circuit also has a rising characteristic which is similar to that of the main signal amplifier and may be determined largely by the proper proportioning of the values of the condenser i| and' resistor II in accordance with well-known principles of design. 7

Two stages of amplification are shown in the feedback amplifier so as to phase the feedback properly for application to the grid of the single stage main amplifierv also simplifies the modulating problem and permits of greater discrimination against the low oscillator frequency ,in the interstage network thereby maklng it-possible to use a greater depth of tremolo without noticeable variations at oscillator frequency in the main output circuit. The

resistor II from the power supply to the cathode of the tube II is used to increase the current flowing in the biasing resistor 31 sonthatthe.

bias on the tube is substantially constant and independent of the variations in plate current. The condenser 88 is used to avoid possible introduction of noise whenope ting the amplitude control II and the-resistor I across the power supply loads the rectifier to improve its regu- The use oi.two stages for steady tones is uniform over the whole frequency rangeto be transmitted.

The tremolo action isessentially a variation in amplitude at a very low frequency such as 5 cycles per second and such a control wave may be obtained in various known ways such as by rotating 8, potentiometer or by using a heterodyne oscillator. consists of a three-stage resistance capacity coupled amplifier with an adjustable feedback circult to form an oscillator of the general type described by Van der Mark and Van der P01 in Physlca, vol. 1, No. 6, nage 437. By proper choice of circuit constants and proper adjustment of the feedback by means of the potentiometer 3|) this circuit maybe made -to oscillate at the desired frequency and this cyclically varying output voltage is applied through the condenser 3| to the reslstor 32. The voltage across the resistor is impressed on the input of the tube H of the feedback amplifier through the resistors 33 and 34.

This cyclic variation in the grid potential of tube II will produce a corresponding variation in the output of the feedback amplifier which in turn produces a low frequency variation or tremolo effect in the output of the main amplifier. The amplitude or depth of the tremolo may be readily controlled from any point convenient to the operator by means of a variable resistor 35 connected across the resistors 32 and 33 to vary the tremolo voltage applied to the tube I In the circuit of Fig. 2 the main signal amplifier comprises a push-pull stage which may be a double triode 4| with conventional input and output circuits except that small condensers 42, 42 or their equivalent, are connected in the circuit externally of the feedback loop to give the amplifier a rising characteristic with frequency as in the case of the amplifier of Fig. l.

The large amount of negative feedback neces sary may be obtained in a single stage by using high gain pentode tubes 43, 44 but it is impractical to apply the high feedback voltage required to the cathodes of the signal amplifier tubes since this would involve an excessive reduction in the effective plate voltage. In order to make it pos sible to apply the feedback to the grids of the tube 4| the proper polarity is obtained, as disclosed in Patent 1,955,827 to Peterson, by crossconnecting the control grids 45 and 46 to the plates 48 and 41, respectively, of' the tube 4|, through the usual resistors 49, 50 and blocking condensers 5| and 52. The plates of the feedback tubes are connected to the grids of the tube 4| through condenser 53, 54 and resistors 55, 56 and these elements and resistors 51 to 6 0 are so proportioned as to give the required rising frequency characteristic as in the circuit of Fig. l. The normal gain of the feedback circuit is fixed at its proper value primarily by the proportioning of resistors, 49, 6| and 50, 62. The plate and screen circuits and the filaments (not shown) are all energizedby means of any suitable rectifier 83.

While the tremolo frequency may be applied by means of the oscillator shown in Fig. 1, it is possible further to simplify the circuit by using for The oscillator shown, however,:

eral sections between the plate and the feedback condenser 65. I

In the circuit of Fig. 1 the permissible amplitude of the tremolo is limited by the discriminacircuit of the feedback amplifier.

tion to the tremolo frequency introduced by the In the circuit of Fig. 2, however, the oscillator frequency is impressedon the mid-branch circuit of the pushpull tubes 43, 44 and is therefore substantially balanced out so that much greater tremolo action can be used before the oscillator frequency appearsin the signal output circuit at sufficient level to be objectionable.

Other possible modifications will occur to those skilled in the art but the invention is intended to be limited only by the scope of the following claims.

What is claimed is:

1. In a sound reproducing system, a source of currents representing sounds to be reproduced, a receiver, an amplifier connecting the source to the receiver, a negative feedback path in the amplifier and means for cyclically varying the transconductance of the feedback path at a subaudio frequency for producing tremolo effects in the output of the receiver.

2. In a sound reproducing system, a source of currents representing sound to be reproduced, a receiver, an amplifier connecting the source to the receiver and having a transconductance which increases with frequency, anegative feedback path for the amplifier having a transmission characteristic which makes the output of the amplifier substantially linear with frequency, a feedback amplifier in the feedback path and means for cyclically varying the transconductance of the feedback amplifier to produce in the receiver a tremolo effect which increases with the frequency of the sounds being reproduced.

3. An amplifying system for producing an arm plitude-modulated signal wave comprising a signal amplifier, a negative feedback path in the amplifier, a push-pull feedback amplifier in the path and means in the-midbranch path of the pushpull amplifier for cyclically varying the gain of the feedback amplifier at a frequency which will produce frequency changes in the output of the signal amplifier.

4. A system according to claim 3 in which the r signal amplifier without feedback and the feedback path each has a transmission characteristic which rises with frequency.

5. In an amplifying system, a signal amplifier comprising two vacuum tubes having grids and plates connected in push-pull, a feedback amplifier comprising two vacuum tubes having plates 

